This invention relates to filters having a porous metallic filter media suitable for the mechanical filtering of both liquids and gases and to a method for their manufacture. More particularly, this invention relates to the bonding of other metal components to a metallic filter media to form a filter assembly.
Metallic filter media typically comprise very finely woven screen, usually utilized in a plurality of superposed layers, or a sintered web or mat of metal fibrils. Often a rather coarse metal screen is placed on either side of a metal fibril web to provide physical protection and to increase the strength of the web. Filters employing metallic media find extensive use in high temperature or corrosive environments.
Construction of filter assemblies containing metallic filter media presents difficult problems in bonding the porous medium to the connecting solid components. It is necessary to obtain a strong, leak-free joint between the parts in order to obtain a useful and reliable filter. The solid components of a filter assembly typically comprise end plates or caps in the case of a filter medium of cylindrical shape and a confining ring or housing in the case of a flat, or pack, filter.
A number of techniques have been proposed and are used to bond metallic filter media to connecting components of a filter assembly. Adhesives such as the epoxies have been used for this purpose, but the resulting filter assembly is limited to relatively low temperature applications.
For high temperature applications, bonding is usually accomplished by brazing, fusion welding or resistance welding. While brazing can produce strong, leak-free bonds, this technique is beset with problems. The metallic filter media, being porous, displays a strong capillary action toward the molten brazing metal. Consequently, brazing metal migrates from the joint area into the filter media reducing the effective filter area and often preventing the formation of uniform fillets. Additionally, the brazing metal is of different composition than that of the filter media and of the other metal components of the filter assembly. In some corrosive environments, this will cause localized electrolytic corrosion and early failure.
Brazing also requires a short time-temperature cycle which effectively rules out furnace brazing techniques. Brazing of stainless steel filter media requires temperatures in excess of the oxidation point thus necessitating use of either an inert atmosphere or a protective flux. If a protective flux is used, it must be subsequently removed and this is an operation which is both costly and unreliable.
Both resistance welding and fusion welding are complicated by the difference in effective thickness between the filter media and the relatively massive metal components to which it is joined. The excessive heat employed by use of these techniques tends to fuse a portion of the filter media, and so reduce its effective filtering area, or to distort the metal components of the filter assembly. Heat distortion is often so extensive as to require machining of the welded assembly to obtain the desired finished dimensions. In addition, the machining step may introduce undesirable metal particulates into the downstream portion of the filter assembly.
Lastly, it has been proposed to attach metal strips over the edges of the filter media by means of crimping or welding and thereafter fusion welding the metal strip to the filter end plates. This technique is disclosed in U.S. Pat. No. 3,426,910. While this joining method produces a reliable seal between the media and the other metal components making up the filter assembly, it does result in the loss of effective filter area particularly in the case of relatively short, cylindrical filters. Moreover, this joining technique is not adaptable for use with a flat, or pack filter.